Solar cells are currently utilized as an energy source by using their ability to convert sunlight to electrical energy. Silicon is used almost exclusively as the semiconductor material in such photovoltaic cells. A significant limitation currently on the use of solar cells has to do with the cost of purifying silicon to solar grade (SG). In view of current energy demands and supply limitations, there is an enormous need for a more cost efficient way of purifying metallurgical grade (MG) silicon (or any other silicon having greater impurities than solar grade) to solar grade silicon.
Companies and research groups have been working on making upgraded metallurgical (UMG) silicon. Many of these processes are limited in that they have difficulty reducing the amount of boron. For example, the process of purifying silicon via an aluminum solvent is ultimately limited by the purity of the aluminum used. The amount of naturally occurring boron in aluminum is relatively low and can be screened from the population of castings at the primary producer's site. This can generally provide aluminum with boron content in the range of 0.6 ppmw. While this is sufficient to produce silicon crystals in the range of 0.4 ppmw, this level of boron is still considered too high to produce solar cells which do not suffer from degraded breakdown voltages. It may be possible to further improve the boron content downstream, but this approach presents its own challenges.